Absorber-crystallizer tower including spray means and scale trap

ABSTRACT

An absorber-crystallizer tower is provided with improved means for spraying aqueous solution into a reactive gas to saturate the solution from which the product crystalizes and with improved means for removing scale from the crystal containing suspension which is formed. The apparatus has no scale sensitive internal surfaces and operates continuously without substantial scale build-up.

BACKGROUND OF THE INVENTION

1. Related Applications

This is a division of application Ser. No. 445,188, filed Feb. 25, 1974,and this application is a continuation-in-part of Ser. No. 213,639,filed Dec. 12, 1971 now abandoned.

2. Prior Art

Absorber-crystallizer towers have been used for years to manufacturevarious products. The present absorber-crystallizer is particularlysuited for the production of sodium bicarbonate by carbonating asolution or mixture of soda ash, but may also be used for production ofother salts, for example, sodium bicarbonate from caustic soda and CO₂,potassium bicarbonate from caustic potash and CO₂, potassium bicarbonatefrom potassium carbonate and CO₂, sodium bisulfite from sodium sulfiteand SO₂, sodium bifluoride from sodium fluoride and HF, diammoniumphosphate from phosphoric acid or monoammonium phosphate and ammonia andother conversions using acidic or basic gases to form crystallizablesalts less soluble than the starting materials. While the presentdescription is directed primarily to carbonation of soda ash to producebicarbonate, it is to be understood that these other chemical processesare suitably carried out in an analogous manner.

In the prior art, one finds many types of absorbers, crystallizers andabsorber-crystallizer combinations. Separate absorber and crystallizerunits connected together with suitable transfer means are commonly inuse. Systems utilizing these separate units have the advantage that eachof the units may be specialized to perform its own particular functionwithout sacrificing efficiency in the other portion of the system.However, such systems also have very substantial disadvantages which todate have not been overcome. The most outstanding disadvantage of suchsystems is that such systems require separate pieces of equipment toperform each individual function. This substantially increases capitalinvestment in equipment but also places great demands on the system foradditional energy which is utilized to transfer liquids from one portionof the system to another. Maintenance costs are likewise increased. Oneof the most serious problems found in this type of system is that theseparate units of necessity provide a great deal of surface area onwhich scale deposits can form. To date no satisfactory means has beenfound for controlling the deposition of scale in such systems. U.S. Pat.Nos. 3,159,456 and 2,895,800, both relating to the crystallization ofammonium salts are typical of this type of apparatus.

It is generally considered desirable and more economical to combine thecrystallization and absorption function into a combinedabsorber-crystallizer. These combined units may also be classified intotwo basic types. The first type is exemplified by U.S. Pat. Nos.2,387,818; 2,424,205 and 2,409,790. In this type a body of liquid,generally saturated, is maintained in a lower portion of theabsorber-crystallizer. A cracker pipe is employed to pass a gas into thegas absorbing body of liquid. The absorption of the gas as it passesthrough the liquid causes supersaturation of the liquid andcrystallization of the desired salt.

While the capital investment in such units is generally less than thatrequired for the individual components, the problem of scale formationis equally severe. This is particularly troublesome at and above theinterface of the liquid and gaseous phases which are present in thistype of unit. As shown in Otto, U.S. Pat. No. 2,424,205 the problem ofscale formation has at least been partially solved by spraying the wallsof the absorber-crystallizer with an unsaturated solution.

The second type of combination absorber-crystallizer is that in whichthe gas to be absorbed exists in a gaseous atmosphere above a body ofliquid and the liquid is passed through the gaseous atmosphere. Typicalof this type of absorber-crystallizer is that shown in U.S. Pat. Nos.2,599,067 and 2,375,922. Again, capital costs are decreased over thatfor the separate units as are maintenance costs. The problem of scaleformation, however, remains.

A second serious problem is also encountered with this type of unit. Todate, no such satisfactory means has been found for spraying the liquidthrough the gaseous atmosphere in large quantities to obtain adequateproduction rates where the concentration of gases contained in thegaseous atmosphere are low. This problem is particularly serious inbicarbonate towers where producers of bicarbonate do not have sources ofgas containing high concentrations of carbon dioxide. Residence time ofthe liquid in the gaseous atmosphere is generally so brief that adequateabsorption cannot take place unless carbon dioxide levels are maintainedat fairly high levels, for example, in the range of 25 - 40 percent.

Applicant's invention relates to the combined absorber-crystallizers ofthe type wherein a liquid is passed through a gaseous atmosphere.Applicant has provided means for not only reducing the amount of scaleformation occurring in the absorber-crystallizer but also for trappingany scale which may be formed to prevent it from either entering intorecycled streams or accumulating in such quantities as to causeblockages in lines exiting from the crystallizer portion of the tower.

Applicant has also provided means for spraying a recycled mother liquorinto the carbonation zone to affect a more efficient absorption of thegas contained therein. Utilizing this spray means one can obtain highcrystal production rates without utilizing high concentration gasmixtures.

SUMMARY OF THE INVENTION

The absorber-crystallizer of this invention comprises a tower ofsuitable diameter in height. It is preferable that the tower besubstantially greater in height than in diameter, preferably in a ratioin about 3:1 to 5:1. The lower portion of the tower, suitably fromone-quarter to one-third or more of the total height is a liquidcontaining classifying crystallizer. This lower portion comprises acrystallization zone which is situated immediately below and contiguouswith an open gas absorption chamber. The lower aspect of thecrystallization zone is defined by a false bottom which is open at itsupper and lower ends. The upper end of the false bottom is sealinglyaffixed to the inner walls of this lower portion of the tower. At leastone wall of the false bottom inclines downwardly and inwardly from theattachment to divide the crystallization zone defined thereabove from acommunicating classification zone between the inclined wall of the falsebottom and the surrounding tower walls. In a cylindrical tower the falsebottom is preferably a frustroconical structure having walls whichincline downwardly and inwardly from a point of attachment on thecylinder walls to terminate in an open lower end. It is alsocontemplated that the false bottom may be one or more inclined wallswhich are affixed to the walls of the tower at the upper end and alongthe sides thereof to separate the crystallization zone thereabove fromthe classification zone between the inclined wall of the false bottomand the tower walls. It is apparent in accordance with the various formsthe false bottom may take that the lower end thereof may be displacedlaterally with respect to the center of the tower or may be locatedcentrally as shown in the preferred embodiment.

A foraminous screen open at its upper and lower ends and extending belowthe false bottom is appended at its upper end to the lower end of thefalse bottom. The foraminous screen is preferably continuous with thelower end of the false bottom and provided with apertures of a sizesuitable to permit passage of crystals into the classification zone butsufficiently fine to restrict passage of oversized materials such asscale which might be present in the mother liquor. The lower opening ofthe foraminous screen communicates with a scale trap positioned beneaththe lower end of the screen. The scale trap is provided with a cleanoutport to permit scale accumulated therein to be purged from the tower.

The upper portion of the tower, suitably the upper two-thirds tothree-quarters comprises a gas absorption zone which is substantiallyfree of surfaces susceptible of scale formation. The gas absorption zoneis thus an open chamber which is immediately above and contiguous withthe crystallization zone previously mentioned. Increased gas absorptionefficiency has been obtained by providing nozzle means spaced in ahorizontal plane about the lower perimeter of the gas absorption zone.Each nozzle so situated is adapted to spray a gas absorbing liquidupwardly and inwardly away from the walls of the gas absorption zone.Each nozzle is functionally opposed to one or more other nozzles so thatthe trajectory of spray eminating from each nozzle is such that opposedsprays collide in mid-air to maximize spray interaction within the gasabsorption zone and to minimize impingement thereof on the walls, tothereby increase gas absorption rates and improve overall productionrates for the absorber-crystallizer.

The invention is further illustrated in the accompanying drawing inwhich:

FIG. 1 is an overall view, partly in section and partly in elevation ofthe absorber-crystallizer tower of this invention;

FIG. 2 is a cross-section through the lower portion of the tower showinga modified false bottom, screen and trap structure which is laterallyoffset from the center of the tower and which may be employed in lieu ofthe frustroconical false bottom illustrated in FIG. 1;

FIG. 3 is a cross-section of the lower portion of the tower illustratinga second embodiment of the false bottom according to the presentinvention in which the false bottom comprises two inclined platesaffixed to the inner walls of the tower.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The absorber-crystallizer of the present invention is a substantiallyvertical tower divided functionally into a lower classifyingcrystallizer which suitably includes that portion of the tower below theliquid level therein and into an upper portion which is an open gasaborption chamber above the liquid level. Suitably the lower portioncomprises from one-quarter to one-third the total height of the towerand the upper portion comprises the upper two-thirds to three-quartersof the total height.

In the preferred embodiment an atmosphere containing at least 4 percentCO₂ is maintained in the upper gas absorption portion of the tower. Aslurry of solid sodium bicarbonate in a solution of sodium carbonatesaturated with sodium bicarbonate is maintained at the lower portion ofthe tower. The lower portion of the tower is divided into acrystallization zone and into a classification zone communicating withthe crystallization zone.

Absorption of CO₂ is achieved by removing slurry from the classificationzone and spraying the same into the gaseous atmosphere in the gasabsorption zone of the tower. To insure adequate CO₂ absorption a highrate or slurry circulation is maintained, suitably from one-tenth toone-half the total slurry volume per minute. Advantageously a pluralityof recycle pumps is useful for transporting the slurry from theclassification zone to the gas absorption zone.

In the gas absorption zone the slurry is sprayed through nozzles intothe gaseous atmosphere. In order to obtain adequate production rateswhere CO₂ levels are low, for example from 4 to 20% CO₂, a specialarrangement and orientation of the spray nozzles is required.Accordingly, it has been found that substantially increased productionrates can be obtained if the nozzles are positioned in a horizontalplane about the perimeter of the lower portion of the carbonation zone.Each such nozzle is positioned or adapted to spray liquid upwardly andinwardly into the gas absorption zone and away from the wall surfacesadjacent each such nozzle. Spray which contacts the walls or theabsorber becomes a part of a flow of slightly unsaturated liquor whichis provided in the upper portion of the gas absorption zone and which issprayed over the roof and walls thereof to prevent scale formation. Thisflow of liquid has little surface area exposed to the gaseous atmosphereand therefor does not participate substantially in the absorption CO₂.Likewise, any contact of the sprays with the walls or with this flow ofunsaturated liquid reduces effectiveness of the sprays in absorbing CO₂both by reducing the sprays' surface area and by substantially reducingits effective exposure or residence time in the gaseous atmosphere.

Applicant has found, however, that CO₂ absorption is substantiallyincreased if each nozzle is oriented in opposing relationship with oneor more other nozzles so that mid-air collision of sprays eminating fromopposed nozzles is maximized. This is most preferably achieved bypositioning the opposing nozzles in radially opposed positions about theperimeter of the gas absorption zone, but a substantially equal effectmay also be achieved between opposed sprays eminating from nozzles whichare not in radially opposed positions.

The effect of the described orientation is two-fold. First, sprayinteraction which occurs between opposed sprays disturbs or interruptsthe lateral movement of each of the sprays and thereby minimizes theamount of each which can reach the opposite wall of the gas absorptionzone. If the spray were permitted to reach the opposite wall, it would,of course, be withdrawn from any substantial participation in CO₂absorption as discussed above. Secondly, the neutralization of radialvelocity vectors of sprays causes resulting upward forces which tend tocarry each droplet in the spray upward in the gas absorption zone. Thiseffectively increases its actual residence time within the absorptionzone and causes a very substantial increase in CO₂ absorption for eachcycle.

Nozzles for introducing the suspension into the carbonating zone maysuitably be either wide angle hollow cone sprays or narrow angle solidcone sprays. For example, commercially available 45° hollow cone spraysgenerally provide more interface area between the gas and liquidcompensating for the reduction and drop residence time due to shortertrajectories. With narrow angle 30° solid cone sprays, residence time islonger but drop size is also larger than for the 45° hollow cone sprays.The reduction in interface due to larger dropsize decreases absorptionof CO₂ in spite of the longer residence time allowed by the narrow anglesprays.

Suitably, commercial spray nozzles having a capacity of 200 gallons perminute operating in the range of 20 - 30 psig provide the required CO₂absorption rate if spray interaction is effected in the manner describedabove. Smaller nozzles will generally produce smaller drops for a givenamount of energy than the larger nozzles but they are also more easilyfouled. For the production of coarse sodium bicarbonate crystals acapacity of 50 gallons per minute represents about the smallest orificesize practically feasible considering that fragments of scale maysometimes appear in the circulating suspension. Hollow cone nozzles arepreferred because they are less subject to blockage by such fragments ofscale than narrow angles solid cone sprays.

An active suspension volume of 30 thousand gallons requires a suspensioncirculation rate of about 10 thousand gallons per minute. Inactivesuspension in the elutriation zone which is the upper portion of thecrystallization zone is about 5 thousand gallons. The total of about 47hundred cubic foot of suspension in a 20 foot diameter tank thusrequires a depth of suspension of about 13.5 feet.

In addition, maintenace of an elutriation zone cross-section of 150square feet (50 percent of the tower cross section) is required tosatisfy the clear liquor needs of a process for producing sodiumbicarbonate crystals. Elutriation velocity is conservatively about onefoot per minute. With 150 square feet elutriation zone this provides aflow of about 1130 gallons of clear liquor. Suitably about 330 gallonsper minute flows through the feed preparation circuit and provides forpurging the system. An additional 600 gallons per minute is used forsuspension fluidization in the classification zone. The remaining 200gallons per minute is used for controlling the flow through theelutriation zone. Thus, for sodium bicarbonate the production a ratio ofabout 3:6:2 of clear liquor divided between the feed circuit,fluidization in the crystallizer and flow through the elutriation zoneis suitably maintained between limits of about ± 20 percent. Withcareful control elutriation velocities up to 2 feet per minute aresuitable, permitting reduction of these rates to a ratio of 3:3:1 orless.

The tower is designed with a steep angled concical roof which is sprayedon its inside surface with a feed solution suitably at rates of theorder of 300 gallons per minute. Advantageously, a hemisphericalcluster-type nozzle is used in this service, but other types aresuitable to provide that a substantial portion of this flow follows theinclined roof and drains along the vertical walls of the gas absorptionzone into the liquid suspension below. This washes away any scale thatoccurs on the walls and also minimizes the tendency for scale to form.The feed solution forming this flow over the roof and walls of theabsorber is preferably a slightly unsaturated solution of sodiumbicarbonate and sodium carbonate which has been recycled from theelutriation zone and fortified with additional sodium carbonate. Sincethis flow occurs as a solid body of fluid as it passes over the walls ofthe absorber, it provides very little surface area which is exposed tothe gaseous atmosphere in the gas absorption zone. Accordingly, it doesnot substantially participate in the absorption of CO₂ from the gasabsorption zone.

The mixture which has been sprayed into the gas absorption zone andbecome supersaturated with respect to CO₂ is collected in acrystallization zone in the lower portion of the tower. The walls ofthis crystallization zone are preferably continuous with the walls ofthe gas absorption zone in order to reduce the amount of scale sensitivesurfaces which are available for scale formation. As indicated above,the lower portion of the tower is divided into a crystallization zoneand a classification zone communicating therewith. The lower aspect ofthe crystallization zone is defined by providing a false bottom which isopen at its upper and lower ends. The open upper end of the false bottomis sealingly affixed to the walls of the tower. At least one wall of thefalse bottom inclines downwardly and inwardly from its point ofattachment and away from the tower walls to divide the crystallizationzone from the classification zone. Suitably, the lower end of the falsebottom may constitute a vertical chimney if desired.

Appended at its upper end to the lower end of the false bottom is aforaminous screen which is open at its upper and lower ends. Theforaminous screen extends downwardly below the false bottom andcommunicates through its lower opening with a scale trap positionedbelow the lower end of the screen. The foraminous screen is alsoprovided with apertures of a size selected to permit passage of slurryinto a classification zone in the peripheral area between the falsebottom, foraminous screen, and scale trap and the outer walls of thetower. Suspension in the classification zone is suitably fluidized byreturning mother liquor or other known means to provide an upperelutriation zone containing relatively fine crystals and/or clear motherliquor and a lower portion containing relatively coarse crystals. If itis desired to grow relatively coarse crystals suspended fines may beremoved from the elutriation zone in an amount sufficient to balancecrystal production rate with crystal growth rate. The growth ofrelatively coarse crystals is further promoted by removing from thelower portion of the crystallization zone a slurry of relatively coarsecrystals which are recycled through the carbonation zone.

Effective scale removal is accomplished by the combined affect ofproviding the internal false bottom screen and scale trap and byrecycling suspended crystals into the gas absorption zone.Desupersaturation is a time related function which is proportional tothe concentration of the crystals in suspension recycled to theabsorption zone. By recycling a concentrated suspension of crystalsrapid desupersaturation is induced, desupersaturation being immediatelyinitiated upon contact of the carbonated spray with the liquor in thecontiguous crystallization zone and being completed before the newlyintroduced suspension descends to the lower end of the false bottom.Consequently, scale formation, also desupersaturation dependent, will beeffectively suppressed before reaching the lower end of the false bottomand other inside surfaces below and beyond this position.

If desupersaturation is incomplete at the bottom end of the falsebottom, as it would be if crystal free mother liquor were sprayed intothe gas absorption zone or if a low concentration of crystals wererecycled, scaling would continue in the openings of the foraminousscreen and in the classification zone and would cause blockages ofproduct removal lines, recirculation lines and of the sprays in the gasabsorption zone, terminating operability of the system until scaleformations were cleared.

Use of Applicant's structure in conjunction with suitable liquid levelsin the tower prevents the occurrence of such blockages and enablesApplicant to operate indefinitely without shutting down for scaleremoval. Scale formation, to the extent it occurs, is completed beforereaching the screen and is thereby prevented from passing into theclassification zone from which product and recycle are removed. Suchscale drops harmlessly into the trap below and is easily removed througha port provided for this purpose.

Applicant has also provided a novel mechanism for providing spray to thegas absorption zone and for clearing any blockages which mightaccidentally occur due to scale formation in this mechanism. At thelower level of the carbonation zone and positioned externally thereof,Applicant has provided tapered circumferential manifolds whichcommunicate with pipes and pumps drawing recycled slurry from theclassification zone. The circumferential manifolds have a tapering crosssection to maintain constant flow velocity to avoid crystalsedimentation as the recycled slurry volume is progressively diminisheddue to diversion of suspension to spray headers extending downwardlytherefrom. A connection at the smaller ends of the tapered manifoldspermits purging with clear liquor to clear any sedimentation ofcrystals. Branching downward from each tapered circumferential manifoldare a plurality, for example 16, liquid conducting connections eachleading to a spray header extending to the interior of the gasabsorption zone through a port in the wall of the tower. Each of theseconnections has in sequence a cut off valve preferably of the plug orball type, a steam connection and a sight glass to permit visual flowmonitoring. The steam connection serves a three-fold function:

1. any obstruction in the spray nozzles is indicated by increased backpressure on the steam gauge on the steam line,

2. the flow of steam through the spray nozzles dissolves minorobstructions in the spray header or nozzle, and

3. by blanking off the spray header at its external flange the steamflow is diverted into the circumferential manifold to clear anyobstruction up-stream from the cut off valve.

An additional isolation valve is advantageously provided to preventfouling of the sensitive steam pressure gauge by entry of saturatedcarbonate solutions when not in use.

The spray headers extending into the gas absorption zone are positionedin a level plane. The hydrostatic head on all of the nozzles is thusequalized and the header is suitably flooded with water or hot dilutesolutions as desired for dissolving obstructions. With such meansprovided for clearing obstructions from the interior of the sprayheaders blockage thereof and curtailment of production is substantiallyavoided. Any intractable blockages are easily corrected by simply andquickly dismounting any individual header and replacing it without anyinterruption or curtailment of production. For this purpose, each sprayheader is supported by a removable port cover. The header assembly isadvantageously supported and guided during assembly and disassembly by atrapezoidal linkage mounted on the exterior wall of the tower whichmaintains alignment between the header and the port at all intermediatepositions. The header is attached to this linkage by a split collarwhich permits simple release and also permits rotation of the header inthe collar. Rotation is advantageous since the sprays are extendedhorizontally within the absorber whereas the ports are arrangedvertically in the absorber walls. A trough immediately beneath theheader ports advantageously serves to collect spillage while the headerport is open. Operation of the tower under slight sub-atmosphericpressure while the port is open prevents excess spillage of thecarbonate solution. With a nozzle pattern not exceeding a three feetspread the clear area of the header ports is suitably 12 inches wide and3 feet high. A trapezoidal linkage with a radial range of 6 feetsuffices to move the headers from a convenient hoist position to themounted operating position not exceeding 4 feet inside the absorberwithout requiring manual positioning in the mounting or dismountingoperations.

Access ports, 3 feet wide and 2 feet high are suitably provided in theroof line of the absorber tower at equally spaced intervals. The accessports provide means for (1) full illumination of inspection of theinterior walls of the tower as desired (2) identification of the areassusceptible to scale formation (3) observing the effect of remedialmodifications in spray patterns to diminish scale formation and (4)establishing correlations between rate of scale formation and otherperformance parameters which serves as a basis for descaling the systembefore accumulations endanger serviceability of equipment. The uppertwo-thirds of the absorber walls is suitably exposed for inspection by atransient reduction in the rate of suspension circulation.

Suspensions of crystals must be maintained in circulation in theclassification zone to avoid crystal sedimentation. The conical bottomof the crystallizer guides the suspension flow into the circulation pumpintakes. Cones with a slope of 2 feet per foot of radius generally avoidsedimentation of crystals on the conical wall. The tip of the cone issuitably truncated to provide a base for a scale trap in the bottom ofthe suspension. The scale trap is suitably formed in the vicinity of thepump intake nozzle by inclined, flat baffled plates isolating a scalecollection zone from the classification zone from which product andrecycle suspension is drawn. A clean-out port suitably about 2 feet wideand 3 to 4 feet high is positioned in the wall to permit access to thescale trap.

As seen in the drawing, the present absorber-crystallizer comprises asubstantially vertical tower having an upper portion above the liquidlevel at "A" comprising an open chamber which serves as a gas absorptionzone 1. The portion of the absorber-crystallizer below liquid level A isthe liquid containing portion of the tower and comprises a classifyingcrystallizer provided with crystallization zone 2 and classificationzone 3. The tower is preferably of cylindrical cross sectionalconfiguration. The cylindrical portion 12 of the tower is surmounted bya steep angle conical roof 11. A gas inlet to gas absorption zone 1 isprovided at 28 and a gas outlet is provided at 13.

Semicircular circumferential manifolds 14 are provided external of thecarbonization zone to receive recycled slurry from classification zone3. The circumferential manifolds are tapered from inlet 15 to thesmaller end 16 to maintain constant flow velocity and to prevent crystalsedimentation therein. A blank connection 17 at the end of taperedmanifold 14 is provided for purging sedimentation from the manifold.Branching downward from tapered circumferential manifold 14, are aplurality of liquid conducting connections 18 communicating with sprayheaders 19 through a port in the cylinder wall. Spray headers 19 arepositioned in a level horizontal plane in the lower portion of gasabsorption zone 1. Each spray header is provided with a plurality ofspaced nozzles 36 which are positioned about the perimeter of the gasabsorption zone. Each spray nozzle is adapted and oriented to sprayliquid upwardly and inwardly into the gas absorption zone to maximizemid-air collision between its spray and the spray eminating from anopposed nozzle. The resulting spray interaction decreases impingement ofthe spray on the walls of the absorber-crystallizer and increases theresidence time of spray droplets in the gas absorption zone. Each of theliquid conducting connections 18 has in sequence a cut off valve 20 asteam out connection 21 a sight glass 22 to permit visual flowmonitoring and a flexible liquid conductor. Port covers 23 supportconnections 18 and spray headers 19 extending into the gas absorptionzone. Trapezoidal linkages 24 provide interim support and guidanceduring assembly and disassembly of connections 18 and maintain thealignment of spray headers 19 and the port at intermediate positions.Attachment of spray headers 19 to linkage 24 is by means of a splitcollar 25 which permits simple release and also permits rotation of theheader in the collar. Trough 26 immediately below the header portsserves to collect spillage while the header port is open. Access ports27 are provided in the conical roof.

The lower portion of the tower comprising the classifying crystallizeris provided with a frustroconical bottom 29 which is continuous with thevertical walls of the cylinder. A false bottom 30 is positioned belowthe liquid level in the tower and is sealingly affixed at its upper endto the walls of the cylinder. False bottom 30 is open at its upper andlower ends and has one or more walls at least one of which inclinesdownwardly from its upper end and inwardly to separate crystallizationzone 2 from classification zone 3. In the preferred cylindrical towershown in FIG. 1 the false bottom is preferably frustroconical but may beof any configuration suitable for separating the crystallization zonefrom the classification zone. For example, it may be a single plateaffixed to the inner walls of the tower as shown in FIG. 2 or a set of 2plates as shown in FIG. 3. False bottom 30 may suitably terminate at itslower end in vertical chimney 31 if desired. Appended at its upper endto the lower end of false bottom 30 is a foraminous screen 32 which isopen at its upper and lower ends and which extends below the lower endof the false bottom. Foraminous screen 32 communicates withclassification zone 3 and through its lower end with scale trap 33 whichin turn communicates to the exterior of the tower through scale cleanoutport 34. The apertures in foraminous screen 32 are of a size selected torestrict passage of oversized crystalline materials into crystallizationzone 3, as these would cause blockages if transported through spraymeans in the gas absorption zone. Pump intakes 35 for recirculatingslurry to gas absorption zone 1 preferably communicate with the lowerportion of classification zone 3.

EXAMPLE

An absorber-crystallizer suitable for producing 50,000 T/yr. of sodiumbicarbonate consists of a tower 20 feet in diameter and 76 feet tall.The bottom 28 foot section is used to crystallize the bicarbonate andthe top 48 feet is used for atmospheric pressure absorption of CO₂. Toensure adequate absorption of CO₂, two pumps are used to provide aslurry circulation rate of 10,000 gpm (gallons per minute). CO₂absorption is achieved by spraying the 10,000 gpm of slurry into the CO₂atmosphere in the tower using 96 nozzles mounted on 16 spray headers.The nozzles are 100 gpm 45° hollow cone nozzles operating at 20 to 30psi. The spray nozzles are located 2 feet above the liquid level in theabsorber and are arranged to direct the sprays upwardly toward thecenter of the tower. Above the level of convengence of the sprays is asecond set of sprays near the base of the top conical section. Thesesprays are directed against the roof to wash it and the walls with warmslightly unsaturated solution.

A funnel-shaped false bottom is sealed internally to the side walls withthe top of the funnel at an elevation of 25 feet above the bottom of thetower and the bottom of the funnel at an elevation of 15 feet. Thebottom of the funnel has a diameter of 12 feet. The concentric zonebetween this funnel and the tower wall provides a quiescent elutriationzone containing crystal free mother liquor and/or relatively finecrystals. Ports fitted with throttle plates near the upper edge of thisbaffle provide for the regulation of the flow of clarified mother liquorupwardly into the crystallization zone in the peripheral zone of thebaffle.

At a level 22 feet above the bottom, a stream of substantially crystalfree mother liquor is removed (outlet not shown) from the elutriationzone and is suitably heated to dissolved suspended crystals and toprovide a solution unsaturated in soda ash in which fresh soda ash isdissolved before return to the roof and walls of the gas absorptionzone. Connections (not shown) for drawing mother liquor with variableconcentrations of fine crystal and nuclei are also provided at anintermediate level of about 17 feet.

The bottom of the absorber-crystallizer has a diameter of 6 feet toprovide space for the trap above the pump intake level. A cleanout portis positioned in the wall for access to the scale trap. The trap isformed of flat baffles approximately 5 feet wide and spaced 6 feet apartat the upper edges. A conical screen with a lower diameter of 5 feetterminates below the top edges of the flat baffles. Scale is retained onthe screen between the baffles. Screen openings are 3/8 inch.

Suspension circulating pump intakes are provided at an elevation of 1foot above the bottom and these discharge in a battery of spray nozzlesat an elevation of 30 feet above the bottom. Fragments of scale toolarge to pass the smallest orifice in the spray nozzles are restrainedby the trap. Recirculated suspension must pass through the screen toreach the pump intakes. Suspension for supplying the crystal centrifugesis drawn from a dynamic suspension zone in the bottom of the tower.

1. In a substantially vertical absorber-crystallizer tower having anupper portion comprising a gas absorption zone, a lower liquidcontaining portion below said gas absorption zone comprising aclassification and crystallization zone, the improvement whichcomprises:a false downwardly and inwardly extending imperforate bottomin said liquid containing portion being open at its upper and lower endsand sealingly affixed at its upper end to the walls of said tower, saidfalse bottom defining a crystallization zone thereabove and aclassification zone between said inclined wall and the walls of saidtower, an outlet from said classification zone, a foraminous screen openat its upper and lower ends communicating between said crystallizationzone and said classification zone, said screen appended to the lower endof said false bottom and extending therebelow and having apertures of asize selected to restrict passage of oversized material therethroughinto said classification zone, the lower opening of said screencommunicating with a scale trap positioned therebeneath, said scale trapbeing provided with a cleanout port to permit accumulated scale to beremoved from the tower, plural opposed nozzles spaced horizontally aboutthe lower perimeter of said gas absorption zone, said nozzles beingarranged and adapted to direct sprays of gas absorbing liquid upwardlyand inwardly into said gas absorption zone, and to avoid spray contactwith said tower walls and to substantially maximize midair collisionwith said sprays eminating from said plural opposed nozzles, gas inletand outlet
 2. The improvement of claim 1 in which said false bottomterminates at its lower end in a substantially vertical chimney to whichsaid screen is
 3. The improvement of claim 1 wherein a plurality ofspray headers extends to the interior of said gas absorption zonethrough a port in the wall thereof, each of said spray headerscommunicating with a plurality of said
 4. The improvement of claim 3wherein a plurality of said spray headers communicate with a taperedcircumferential manifold positioned externally
 5. The improvement ofclaim 4 wherein each of spray headers is positioned for lateral movementon a trapezoidal linkage affixed to the outer wall of said tower, eachsaid linkage being adapted to recipricate a corresponding spray headerfrom an operating position within said tower to a repair
 6. Theimprovement of claim 1 wherein said false bottom is frustroconical.